In recent years, battery-driven mobile terminals having a communication function such as smart phones, tablets (computers), and the like, need to be communicated by reducing the consumption of wasteful power. Therefore, in some cases, a drain modulation system such as Envelope Tracking (ET), polar modulation, Envelope Elimination and Restoration (EER), and the like is adopted as a power amplifier (PA) which is one of the blocks having a largest power consumption in a mobile terminal.
As such power supply device of an EER system and an ET system employing a drain modulation system, for example, a hybrid-type power supply device in which a linear regulator and a switching regulator operate in collaboration with each other is used.
In other words, such power supply device is used as a power supply of a power amplifier for high frequency signals such as a transmitter. A power voltage of the amplifier is increased or decreased in response to a change of an envelope component drawn by a peak value of an input signal to reduce wasteful power and enhance efficiency.
As described above, a hybrid-type power supply device in which a linear regulator and a switching regulator operate in collaboration with each other has been put into practice, and such power supply device commonly employs a current detecting resistance for a circuit monitoring an output current of the linear regulator.
In other words, since the hybrid-type power supply device commonly detects the output current of the linear regulator by using the current detecting resistance, a power consumption resulting from the current detecting resistance causes degradation in power efficiency of the power supply device or the power amplifier.
In this regard, various types of hybrid-type power supply devices in which a linear regulator and a switching regulator operate in collaboration with each other have been proposed.    Patent Document 1: Japanese National Publication of International Patent Application No. 2010-508577    Patent Document 2: Japanese Laid-open Patent Publication No. 2012-134705    Non-Patent Document 1: Feipeng Wang, et al., “Design of wide-bandwidth envelope-tracking power amplifiers for OFDM applications,” IEEE Microwave Theory and Techniques Society, pp. 1244-1255, April 2005    Non-Patent Document 2: Feipeng Wang, et al., “An Improved Power-Added Efficiency 19-dBm Hybrid Envelope Elimination and Restoration Power Amplifier for 802.11g WLAN Applications,” IEEE Microwave Theory and Techniques Society, pp. 4086-4099, December 2006